C Programming Program: malloc, free, pointer in C char *str= “this is a test onl

Question

C Programming

Program: malloc, free, pointer in C

char *str= “this is     a test only –just a test”; /* or other string */

char *ptr1[100];

char *ptr2[100];

– Parse strwith a pointer, and set ptr1 to each word (NULL terminated) within the str(i.e., skip white spaces)

             • ptr[0] points to “this”, ptr[1] points to “is”, ptr[2] points to “a”, etc

– Loop: print ptr1[i] that points to valid word

– Loop: for each ptr2[i], mallocand copy the string from ptr1[i], print ptr2[i]

– loop: free each of ptr2[i]

Explanation / Answer

Functions used in the program:

strtok(str , delimiter): To parse the string str separated by a delimiter.

strncpy(str1 , str2 , len2): Copy to the destination string (str1) the values of source string (str2) upto first len2 elements.

malloc(): Allocate the memory to the pointer.

free(): Free the memory allocated using malloc().

Follwing code is explained with the comments inside the code.

----------------------------------------------------------------------

C Code:

#include <stdio.h>
#include <string.h>
#include <malloc.h>

int main(){

// Test string
char *str="this is     a test only –just a test";
char buffer[256]; // some buffer
char *ptr1[100];
char *ptr2[100];
int i=0,j=0;
char *temp;

strcpy(buffer,str); // copy the str contents to the buffer

temp=strtok(buffer," "); // get the first token

while(temp!=NULL){ // while we have a valid token
    ptr1[i]=temp; // ptr1[i] points to the token
    i++;
    temp=strtok(NULL," "); // get next token
}

j=i;
i=0;

// print the values pointed by ptr1[i]
while(i<j){
    printf("ptr1[%d] = %s ",i,ptr1[i]);
    i++;
}

printf("------------ ");

i=0;

// allocate the memory to ptr2[i] and copy the contents of value pointed by ptr1[i]
while(i<j){
    ptr2[i]=(char *)malloc((strlen(ptr1[i])+1)*sizeof(char));
    strncpy(ptr2[i],ptr1[i],strlen(ptr1[i]));
    i++;
}
ptr2[i]=NULL;

i=0;

// print the contents of ptr2[i]
while(ptr2[i]!=NULL){
    printf("ptr2[%d] = %s ",i,ptr2[i]);
    i++;
}

// deallocate the memory

for(j=0;j<i;j++){
    free(ptr2[j]);
}

return 0;
}

----------------------------------------------------------------------

OUTPUT:

ptr1[0] = this
ptr1[1] = is
ptr1[2] = a
ptr1[3] = test
ptr1[4] = only
ptr1[5] = –just
ptr1[6] = a
ptr1[7] = test
------------
ptr2[0] = this
ptr2[1] = is
ptr2[2] = a
ptr2[3] = test
ptr2[4] = only
ptr2[5] = –just
ptr2[6] = a
ptr2[7] = test

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